Gravity exerts an earth-bound force on a fired projectile, causing the projectile to depart from its line of fire and strike lower than aimed. Therfore, when a shooter aims at a target, the effect of gravity must be overcome by aiming higher than the actual target. The total effect of gravity in pulling a projectile out of its line of fire is a function of the horizontal, not straight-line, distance from shooter to target. In other words, the farther away horizontally the target, the higher the shooter must aim above the target to compensate for gravity. Most rifles allow the shooter to xe2x80x9czeroxe2x80x9d the sights based on the distance to target the shooter knows the straight-line distance to the target, and sets the sights according to that distance, which results in aiming above the target, thereby compensating for gravity.
This system works well when the shooter and target are at the same elevation because the horizontal distance and the straight-line distance are the same. Therefore, even though a shooter should set his rifle sights according to the horizontal distance to target, and not the straight-line distance, when those two distances are equal, knowing one is as good as knowing the other. However, if the shooter is required to aim upward or downward to engage his target, setting the sights according to the straight-line distance to target will result in the projectile striking too high. The shooter must calculate the horizontal distance to target to compensate for gravity""s effect. Traditionally, a shooter would measure the vertical angle between himself and his target by means of a protractor, calculate the cosine of the angle, multiply the straight-line distance by the cosine to get the horizontal distance, and set his sights according to that distance. This method is time-consuming and requires the shooter to make an accurate trigonometric calculation before firing. This is especially problematic when time is of the essence in sighting, engaging, and firing at the target.
It Is an object of the present invention to provide a small, lightweight device providing a faster, more reliable method for correcting for the gravitational effect when shooter and target are at different elevations. This is accomplished by allowing the shooter to read the cosine of the angle directly without breaking his cheek weld with the rifle, thereby removing the need to make a trigonometric calculation.
The present invention""s casing attaches directly to the rifle or scope by means of a mounting apparatus that is part of the casing, and remains fixed with the rifle so that it rotates as the rifle rotates. The casing houses an inner wheel upon which a marker is inscribed. The wheel is free to rotate independently of the casing and a stabilizer comprised of a weight keeps the wheel fixed relative to horizontal, while the casing rotates with the gun as the shooter aims up or down.
A window on the casing allows viewing of the inner wheel, and has indicia comprised of cosine numbers inscribed on it. As the shooter aims the rifle up or down, the cosine numbers rotate relative to the marker on the inner wheel, and the cosine number aligning directly over the marker represents the cosine of the angle at which the rifle is currently positioned. The shooter can then multiply the straight-line distance to target by the cosine number. This gives the horizontal distance to target, which is the number required for adjusting the rifle""s sights before firing.